There have been proposed various exhaust emission purifying apparatuses in each of which a liquid reducing agent or its precursor is injection-supplied to an exhaust emission upstream side of a NOx reduction catalytic converter disposed in an engine exhaust system, so that NOx in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction, to thereby reductively purify NOx into harmless components. In the exhaust emission purifying apparatus, when an injection nozzle which injection-supplies the liquid reducing agent or its precursor is clogged, since an appropriate amount of reducing agent is not supplied to the NOx reduction catalytic converter, a function as the exhaust emission purifying apparatus cannot be achieved. Therefore, as disclosed in Japanese Laid-Open (Kokai) Patent Application Publication No. 2006-132442 (Patent literature 1), there has been proposed a technology for indirectly judging whether or not the injection nozzle is clogged, based on various types of parameters.
However, as factors that the appropriate amount of reducing agent is not supplied to the NOx reduction catalytic converter, there can be assumed various factors, for example, not only clogging of the injection nozzle but also clogging or cracks of piping, a failure in an electronic control system or the like. Therefore, a technology for performing not only the clogging judgment of the injection nozzle but also the comprehensive failure diagnosis on a supply system of the liquid reducing agent or its precursor is necessary.